Two studies published in the journal Archives of Internal Medicine have called into question the extent and safety of computerized tomography (CT) scans. First introduced to the U.S. in 1974, there are now around 6,000 CT scanners that perform more than 70 million scans, up from three million in 1980. Few have questioned the CT’s diagnostic power. The scanners have revolutionized doctors’ ability to see diseases that previously could only be found with laparoscopy or at autopsy. Indeed, the inventors of the CT scanner were awarded the Nobel Prize for medicine. But some are concerned about the high levels of radiation that may accompany some of these scans.

Each day more than 19,500 CT scans are performed in the U.S., exposing patients to the equivalent of 30-442 chest X-rays. Just one CT coronary angiogram delivers on average the equivalent of 309 chest radiographs. Radiation doses for pelvic-abdominal CTs averaged 50% greater than for coronary studies.

Although most patients receive relatively low radiation exposure, 20% of patients studied had moderate exposure, and around 2% were exposed to high or very high doses of radiation. Even the average dose of CT scanners studied delivered four times the expected dose for such tests.

Researchers reported on average a 13-fold variation in radiation exposure for each CT type across the centers that were studied. This unsettling finding points to the wide variability in radiation between CT scan protocols as well as between machines. One CT scan isn’t necessarily the same as another with respect to quality of scan or degree of radiation exposure.

With such a large range of exposures among millions of patients, researchers set out to estimate the cancer risk from CT scan exposure. Taking into account the age, sex and CT study type, researchers estimate that one in every 270 forty-year-old women undergoing a CT coronary angiogram will develop some form of cancer from the procedure in her lifetime.

Next, the study authors set out to estimate the potential number of cancers that could be caused by radiation exposure from CT scans from an examination of insurance records. If scans conducted after a cancer diagnosis and those performed in the last five years of life are excluded, 29,000 excess cancers could result from the CT scans done in 2007 alone.

These cancers are expected to appear in the next 20-30 years, and if you accept the authors’ estimate of a 50% mortality rate, one-half or 15,000 can be expected to eventually die from the radiation exposure. Two-thirds of the projected cancers will occur in women, and 15% will develop from scans done in children and teens. Researchers caution that an additional 2,000 cases of breast cancer may occur just from the CT scans done in 2007.

Keep in mind that these cancer deaths are estimates from data sets and not directly measured or verified. Moreover, the study’s projection of a 50% mortality rate from the predicted cancers seems very aggressive. Overall, cancer survival for all tumor types has improved from 50% in the 1970s (just at the advent of CT scanning) to over 66% for all cancers during the time period 1996 to 2004. Cancer incidence rates have fallen during that same timeframe as well. If CT scanners were generating such a large volume of tumors with high death rates, current epidemiological measures do not yet reflect it.

So are the studies a true cause for alarm or just the latest round of doom-saying about the perils of modern medicine? Perhaps the greatest cause for concern is the variability detected between protocols and CT scanners for similar studies. Imaging centers should all re-examine their technologies and protocols with an effort to minimize radiation exposure and carefully assess the need for a scan. Patients need to be better educated about the potential risks and benefits.

More discretionary scans, such as whole body scans to look for occult disease, should be re-evaluated in light of the study findings. If consumers knew the potential risks from the CT study, they might be less inclined to satisfy their curiosity. Importantly, the manufacturers are known to be working on lower-dose CT scanners that will help to minimize radiation exposure.

If medically indicated, CT scanners still can be life-savers by improving the quality and timeliness of diagnosis for certain conditions. Patients and their physicians should not hesitate to employ them after full consideration of their potential risks and benefits.